The global epidemic of AIDS has created an urgent need for a vaccine against human immunodeficiency virus type 1 (HIV-1). It is likely that effective AIDS vaccines will need to generate efficient humoral and cellular immune responses. Virus-neutralizing antibodies and anti-HIV cytotoxic (CD8+) T lymphocytes (CTLs) mediated immunity are major requirements for protective immune responses elicited by HIV vaccines.
HIV has several major genes coding for viral proteins. The gag gene codes for p24, the viral capsid; p6 and p7, the nucleocapsid proteins; and p17, a matrix protein. The pol gene codes for reverse transcriptase, integrase, and protease which cleaves the proteins derived from gag and pol into functional proteins. The env gene codes for the precursor to gp120 and gp41, envelope proteins embedded in the viral envelope that enable the virus to attach to and fuse with target cells. The tat, rev, nef, vif, vpr, vpu genes each codes for a single protein with the same names, Tat, Rev, Nef, Vif, Vpr, Vpu, respectively.
Neutralizing antibodies have been shown to contribute to protection from virus infection in animal models of HIV-1 infection. The virus-specific targets on HIV-1 accessible to neutralizing antibodies are the envelope glycoproteins (Yang, X. et al. (2005) “Stoichiometry of Antibody Neutralization of Human Immunodeficiency Virus Type 1” Journal of Virology 79: 3500-3508). During the normal course of HIV-1 infections, virus-neutralizing antibodies are often generated but the titer of neutralizing is often low. Most neutralizing antibodies bind the gp120 envelope glycoprotein, which is the major exposed protein of the viral envelope glycoprotein trimer. The more conserved receptor-binding surfaces of the HIV-1 gp120 glycoprotein are also the targets for neutralizing antibodies. The CD4-binding site (CD4BS) antibodies recognize a conformational epitope composed of several segments of gp120 region that overlaps the binding site for CD4. CD4-induced (CD4i) antibodies bind a highly conserved gp120 element that is critical for the gp120-chemokine receptor interaction. The ability of CDBS and CD4i antibodies to interfere with receptor binding contributes to their neutralizing capability.
GP 120 contains ten domains: conserved domains 1-5 (C1-C5) and variable domains 1-5 (V1-V5). The C1 and C5 domains are located at N- and C-terminals of gp120, respectively. Antibodies directed against the V3 loop, which determines chemokine receptor choice, can block the binding of gp 120 to the receptors CCR5 and/or CXCR4. Neutralization by anti-V3 antibodies, although potent, is often limited to a small number of HIV-1 strains.
Gp120 is non-covalently associated with gp41. The gp41 subunit is anchored in the membrane and has a non-polar fusion peptide at its N-terminus. The gp120-gp41 complex forms oligomers on the surface of infected cells and on virions. The binding of gp 120 to CD4 is thought to result in activation of the membrane fusion activity of gp41, leading to entry of the viral nucleocapsid into a cella Antibodies to gp41 epitopes in the serum of HIV-infected individuals may play an important role in virus neutralization. Gp120-41 complex sequences of different HIV subtypes show a remarkably conserved N-terminal coiled-coil structures of gp41 as well as the C-terminal residues that interact with the N-terminal core structure of gp120.
Multiple immune effectors participate in prevention, containment and clearance of HIV infection. To prevent infection of host target cells, antibodies are required. After the first target cells have been infected with virus, it is important to have cytotoxic T lymphocytes (CTLs) as well as antibodies to reduce cell-to-cell spread and kill infected cells. An effective HIV vaccine should evoke antibodies that can bind to virus and prevent attachment of virus to target cells, as well as CTLs that can eliminate any cells that become infected.
It remains a difficult goal for vaccinologists to construct live-attenuated viruses that are both effective and safe, or to mimic the presentation of viral proteins observed in infection with recombinant antigens or with replicating or non-replicating vectors carrying appropriate genes or antigens. The large number of mutations in the V3 domain of gp120 has limited its usefulness as a target for HIV vaccine. It is still unclear how the trend of hypervariability in the variable domains is developing and how many domains are absolutely invariant in the evolving strains of HIV.
A previously unaddressed need exists in the art to address the deficiencies and inadequacies in HIV vaccine antigen production, especially in connection with the provision of efficacious, antigenic determinant peptides.